This invention relates to switching apparatus for alternating current electrical circuits, and more particularly, to control circuits for turning on a solenoid or the like for one half cycle of a predetermined alternating current cycle which occurs at least one full cycle after the circuit receives an operator command to operate the solenoid.
Solenoids for electric staplers and other devices are generally controlled by a mechanical switch which may be actuated by an operator or other machine apparatus. The switch is part of a solenoid control circuit which only permits the solenoid to operate for a predetermined interval of time each time that the mechanical switch is actuated. The solenoid is turned off by the control circuit at the end of the interval, even if the switch is still actuated, and cannot be operated again until the mechanical switch is released and actuated again.
Commercially available solenoids operate through an entire stroke on one full half-cycle of line current, at about 110 volts and 60 Hz. Some known solenoid control circuits include a silicon controlled rectifier (SCR) or other device in series with the power source and solenoid for controlling the solenoid so that current only flows for no longer than one full half cycle. Such circuits generally turn the SCR on immediately upon actuation of an operator controlled mechanical switch. The performance of such circuits is somewhat limited because if the SCR is turned on toward the end of a half-cycle, when the input current is near zero, adequate power may not be provided to the solenoid for proper operation.
Other control circuits delay turning the SCR on after the mechanical switch is actuated until shortly after the current crosses zero and enters the first full positive half-cycle of current immediately following the actuation of the switch. In this manner, the solenoid receives current for substantially all of the half-cycle, regardless of when the mechanical switch is actuated.
The contacts of most mechanical switches have some tendency to bounce when actuated, which may cause improper operation of the solenoid. The zero-crossing circuits just discussed may not eliminate the undesirable effects of contact bounce because the maximum delay is less than a full cycle, and the contacts may bounce after the delay. Thus, there is a need for control circuits for solenoids and the like which provide a delay of at least one full cycle between the time that the mechanical switch is actuated and the time that the SCR turns on the solenoid.
In many previous control circuits, power is permanently connected to the solenoid through the SCR or other switching device without a mechanical switch which physically breaks the power line when the solenoid is not in operation. If the SCR is turned on by transients, the solenoid may operate unexpectedly, and if the SCR shorts, the solenoid may remain on indefinitely. Both of these conditions are undesirable. Thus, there is also a need for a control circuit for the solenoid of electric power staplers and the like which mechanically interrupts the power to the solenoid when the solenoid is not operated.